1. Field of the Invention
The present invention relates to timing belt tensioners and more particularly to adjustable timing belt tensioners.
2. Prior Art
In an overhead cam shaft engine or a high cam shaft engine which utilizes a timing belt, a timing belt tensioner is provided between the crank shaft pulley and the cam shaft pulley for the purpose of eliminating any slack in the timing belt.
In the prior art there exists timing belt tensioners such as that shown in FIG. 1. Timing belt tensioner of FIG. 1 includes a base plate 1 to be mounted on the front surface of an engine cylinder block (not shown). One half of the base plate 1 comprises a plate and the other half comprises a hub 1A formed thereon for holding a tensioner bearing 2. The plate portion has a hole 1a and a slot 1b provided therein. The slot 1b is for adjusting the position of the base plate 1 when mounting it to the front surface of a cylinder block (not shown), and for inserting a tightening bolt (not shown) therein. Slot 1b is an arcuous slot whose center of curvature is the center of the hole 1a.
To mount the base plate 1 to the cylinder block, first a tightening bolt is inserted through hole 1a into a screw hole provided in the front surface of the cylinder block. Next, using a spring, the tension bearing 2 on hub 1A is pressed against the timing belt (not shown) at a desired pressure. Finally, a second tightening bolt is inserted through slot 1b into the screw hole provided in the front surface of the cylinder block. However, before tightening the bolt which has been inserted into slot 1b, the base plate must be adjusted to a mounting position by rotating the plate at a desired angle parallel with the plate surface and with hole 1a at the center of rotation. The bolt in arcuous slot 1b is then tightened into the screw hole to firmly hold the base plate 1 in place.
The primary problem of the prior art timing belt tensioners of FIG. 1 is that since the mounting position of plate 1 and the position of plate 2 are at a distance and a force is applied to bearing 2, the force applied to bearing 2 is transmitted back to base plate 1 as twisting and bending moments thereby causing base plate 1 to bend slightly. As a result, the axial line of the tensioner bearing cannot maintain a parallel relationship with the timing belt surface thereby causing the timing belt to become slightly twisted. As a final result, since the timing belt is slightly twisted it will generate vibrations while the engine is in operation thereby shortening the life of the timing belt.
A second problem with the prior art tensioner of FIG. 1 is that in order to prevent base plate from being slightly bent, the base plate must be made stronger by increasing the thickness of the plate. The resulting increase in thickness and accordingly weight, necessitates enlargement of the installation space on the cylinder block as well as an increase in the cost of manufacturing of the timing belt tensioner. A particularly undesirable aspect associated with such a large installation space is that various problems arise in connection with the designing of the placement of other parts which must be installed on the front surface of the cylinder block.